Biosensing well array with protective layer

ABSTRACT

The present disclosure provides a biological field effect transistor (BioFET) and a method of fabricating a BioFET device. The method includes forming a BioFET using one or more process steps compatible with or typical to a complementary metal-oxide-semiconductor (CMOS) process. The BioFET includes a microwells having a sensing layer, a top metal stack under the sensing layer, and a multi-layer interconnect (MLI) under the top metal stack. The top metal stack includes a top metal and a protective layer over and peripherally surrounding the top metal.

PRIORITY CLAIM

This application claims the benefit to and is a divisional of U.S.patent application Ser. No. 14/033,089, filed on Sep. 20, 2014 andentitled “BIOSENSING WELL ARRAY WITH PROTECTIVE LAYER” which applicationis incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to biosensors and methods for formingbiosensors. Particularly, this disclosure relates to biologicalfield-effect-transistors (bioFETs) and methods for forming them.

BACKGROUND

Biosensors are devices for sensing and detecting biomolecules andoperate on the basis of electronic, electrochemical, optical, andmechanical detection principles. Biosensors that include transistors aresensors that electrically sense charges, photons, and mechanicalproperties of bio-entities or biomolecules. The detection can beperformed by detecting the bio-entities or biomolecules themselves, orthrough interaction and reaction between specified reactants andbio-entities/biomolecules. Such biosensors can be manufactured usingsemiconductor processes, can quickly convert electric signals, and canbe easily applied to integrated circuits (ICs) andmicroelectromechanical systems (MEMS).

Biochips are essentially miniaturized laboratories that can performhundreds or thousands of simultaneous biochemical reactions. Biochipscan detect particular biomolecules, measure their properties, processthe signal, and may even analyze the data directly. Biochips enableresearchers to quickly screen large numbers of biological analytes insmall quantities for a variety of purposes, from disease diagnosis todetection of bioterrorism agents. Advanced biochips use a number ofbiosensors along with microfluidics to integrate reaction, sensing andsample management. BioFETs (biological field-effect transistors, orbio-organic field-effect transistors) are a type of biosensor thatincludes a transistor for electrically sensing biomolecules orbio-entities. While BioFETs are advantageous in many respects,challenges in their fabrication and/or operation arise, for example, dueto compatibility issues between the semiconductor fabrication processes,the biological applications, restrictions and/or limits on thesemiconductor fabrication processes, sensitivity and resolution of theelectrical signals and biological applications, and/or other challengesarising from implementing a large scale integration (LSI) process.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a cross-sectional view of a BioFET according to one or moreembodiments in accordance with the present disclosure.

FIGS. 2A and 2B are flow charts of method embodiments of fabricating aBioFET device according to one or more aspects of the presentdisclosure.

FIGS. 3 to 12 are cross-sectional views of a portion of a workpiece atvarious intermediate stages of forming a BioFET according to one or moreaspects of the present disclosure.

FIG. 13 is a flow chart of a method embodiment of using a BioFET deviceaccording to one or more aspects of the present disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Further still, references to relative termssuch as “top”, “front”, “bottom”, and “back” are used to provide arelative relationship between elements and are not intended to imply anyabsolute direction. Various features may be arbitrarily drawn indifferent scales for simplicity and clarity.

In a biological field-effect transistor (BioFET), the gate of ametal-oxide-semiconductor field-effect transistor (MOSFET), whichcontrols the conductance of the semiconductor between its source anddrain contacts, includes a bio- or biochemical-compatible layer or abiofunctionalized layer of immobilized probe molecules that act assurface receptors. Essentially, a BioFET is a field-effect biosensorwith a semiconductor transducer. An advantage of BioFETs is label-freeoperation. Specifically, using BioFETs can avoid costly andtime-consuming labeling operations such as the labeling of an analytewith, for instance, fluorescent or radioactive probes.

Binding of a target biomolecule or bio-entity to the gate or a receptormolecule immobilized on the gate of the BioFET modulates the conductanceof the BioFET. When the target biomolecule or bio-entity is bonded tothe gate or the immobilized receptor connected to the gate, the draincurrent of the BioFET is varied by the gate potential, which depends onthe type and amount of target bound. This change in the drain currentcan be measured and used to determine the type and amount of the bondingbetween the receptor and the target biomolecule or the biomoleculeitself. In some embodiments of different circuit design, the devicecould work in linear or saturation region of the IV curve forbiosensing. A variety of receptors may be used to functionalize the gateof the BioFET such as ions, enzymes, antibodies, ligands, receptors,peptides, oligonucleotides, cells of organs, organisms and pieces oftissue. For instance, to detect ssDNA (single-stranded deoxyribonucleicacid), the gate of the BioFET may be functionalized with immobilizedcomplementary ssDNA strands. Also, to detect various proteins such astumor markers, the gate of the BioFET may be functionalized withmonoclonal antibodies.

One example of a biosensor has a sensing surface over a top metal plateconnected to the gate of the BioFET. The metal plate and the sensingsurface is a floating gate for the BioFET. The floating gate isconnected to the gate structure of the BioFET through several layers ofmetal interconnect lines and vias (or multi-layer interconnect, MLI). Insuch a BioFET, the potential-modulating reaction takes place at an outersurface of the metal plate or a dielectric surface formed on top of themetal plate. Each microwell, through different sensing layers and metalplates, is connected to a different transistor. The microwell is formedover the top metal plate for each BioFET. The microwells are isolatedfrom each other, and the reactions take place on the sensing layer ineach microwell. The various microwells are connected by microfluidicchannels. Reagents are flowed through the microfluidic channels to eachsensing layer in the microwells. The reagents include test samples thatdirectly bind to the sensing layer or indirectly through a carrier. Anexample of a carrier is a bead having the test samples bound thereon. Inone example, the binding reaction changes a local ion concentration (pH)in a microwell that causes a change in the internal charge of thesensing layer. The charge of the sensing layer is transmitted to thetransistor gate through the various metal layers as a voltage signal.The change in gate voltage changes the amount of current flowing betweenthe source and drain of the BioFET. By detecting the current, the changein pH in the microwell is measured. Size of the microwells is directlyrelated to the signal intensity. Larger microwells allow a largersensing layer/more bio-entities that can include more binding sites tocreate a stronger signal.

The top metal plate is prone to corrosion if exposed to the analyte;such corrosion would render the BioFET defective. While the sensinglayer protects the top metal plate from the analyte, a bottom of themicrowell is sized to be smaller than the top surface of the top metalplate within alignment tolerances to further ensure that the top metalplate is not exposed to the analyte. In other words, the bottom of themicrowell is sufficiently small such that even with misalignment, themicrowell would still be situated over the metal plate. Adequate spacingis maintained between adjacent top metal plates to isolate themicrowells from each other as well as following the design rules for thetop metal electrodes.

An increase in biochip capacity is desirable to allow more simultaneousreactions and more accurate measurements. Higher biochip capacityinvolves building more transistors and a higher number of correspondingmicrowells. Having more microwells reduces the area of each microwell,as only a finite space is available on the biochip. When the size of themicrowells decreases, the area of the sensing layer also decreases,which decreases signal intensity and increases signal-to-noise ratio(SNR).

One way to minimize the decrease in signal intensity involves preservingthe sensing layer area as the number of microwells increase. In someexamples, microwells having bottoms larger than the top metal plate areused. The larger microwell bottom increases the sensing layer area.While the microwells are larger, a gap between the top metal plate andthe passivation wall may be created that are filled by the sensinglayer. A misalignment between the microwell and the top metal plate cancreate a crack corrosion site and render the transistor defective. Withmetal sensing layers, the likelihood that the sensing layer bridges tothe top metal of an adjacent microwell increases when there is amisalignment. Therefore, having microwells with bottoms larger than thetop metal makes misalignment window very small for the microwell and topmetal electrode.

Other examples to increase the sensing layer area involve adding asmaller metal plug over the top metal electrode and a sensing plate overthe smaller metal plug. The sensing plates may be placed closer than thetop metal plates and thereby increase the area of the microwells. Havinga sensing plate over the smaller metal plug reduces the likelihood ofbridging signals between adjacent microwells. However, adding a smallermetal plug and a sensing plate having different dimensions adds twoadditional layers with two photomask patterns that increase themanufacturing cost significantly.

The present disclosure pertains to a method and structure for formingmicrowells that are larger as compared to the microwells over the topmetal plates without misalignment issues. According to variousembodiments, a top metal stack includes a protective layer over andperipherally surrounding the top metal to ensure separation of theanalyte and the top metal material. A sensing layer is deposited in themicrowells and over the field. At least the field portion of the sensinglayer is removed in an etch process while a photomask protects theportions of the sensing layer within the microwells. The removal of thefield portion of the sensing layer isolates the microwells from eachother.

FIG. 1 is a cross-sectional view of a BioFET 100 according to one ormore embodiments in accordance with the present disclosure. The BioFET100 includes a substrate 103 on and in which a transistor is formed. Thesource and drain regions 105 are formed in the substrate 103. A gatestack including gate dielectric 107 and gate electrode 109 is formed onthe substrate 103. As shown in FIG. 1, the transistor in BioFET 100 is aplanar transistor; however, other types of transistors may be used,including a multi-gate transistor or a FinFET. The BioFET 100 alsoincludes a gate contact 111 over the gate electrode 109. Contacts to thesource and the drain (not shown) are also included. A number of metalinterconnect layers 113 interpose between the gate contact 111 and amicrowell 101. Each metal interconnect layer 113 includes a metal line115 and metal via 117 within a layer of intermetal dielectric 119. Threemetal interconnect layers 113 are shown, but fewer or more may be used.

The microwell 101 is an opening in the passivation layer 125 andincludes a sensing layer 121 on the bottom and at least a portion of thesidewalls. Having sensing layer 121 on the sidewalls increases thesurface area of the sensing layer 121. According to various embodiments,the sidewalls may not be fully covered by the sensing layer 121. Thesensing layer 121 may be a metal, dielectric, or a polymer. Examplesinclude titanium nitride, high-k dielectric such as aluminum oxide,lanthanum oxide, hafnium oxide, and tantalum oxide.

A top metal stack 127 is disposed between the last top metalinterconnect layer 113 and microwell 101. The top metal stack 127directly contacts the sensing layer 121 of the microwell 101 and themetal via 117 of the last top metal interconnect layer 113. The topmetal stack 127 includes a top metal 129 and a protective layer 131 overand peripherally surrounding the top metal 129. The top metal stack 127may also include an adhesion layer under the top metal 129 and anantireflection layer between the protective layer 131 and the top metal129.

A BioFET device includes a number of BioFETs 100 with microwells 101that are in fluidic communication with each other. Each microwell 101 isassociated with gates of one or more transistors. When a microwell 101is connected to the gates of more than one transistor, a higherfrequency sampling may be performed to increase the accuracy of themeasurement. The microwells are connected by microfluidic channelsforming an array of BioFETs 100. The microfluidic channels allow ananalyte to flow from an inlet of the BioFET device to an outlet of theBioFET device. The microfluidic channels may be above the microwells 101as shown in FIG. 1 or be at a same level as the microwells 101. Theanalyte includes test samples and a carrier medium. In some embodiments,the test samples include functionalized beads 133 on which specificbiomolecules 135 from the test samples would bind. The functionalizedbeads are sized such that only a particular numbers of them would fit ina microwell. For example, the functionalized beads 133 may be slightlysmaller than a microwell such that only one bead 133 can fit in amicrowell. The biomolecules 135 on the functionalized bead 133 wouldchange the fluidic environment in the microwell 101 in a way that isdetectible by the transistor. In other embodiments, the test sampleincludes biomolecules 137 that would bind to receptors labeled on thesensing layer 121 without using carrier beads. For example, singlestranded deoxyribonucleic acid (ssDNA) is bound on the sensing layer andamplified with PCR (polymarse chain reaction) to duplicate the same DNAto increase sites. Then, reagent is flowed through the microwells forDNA sequencing. Other examples include protein labeling andanti-body/anti-gen reactions.

FIG. 2A is a method 200 of fabricating a BioFET device according to oneor more aspects of the present disclosure. The method 200 begins atoperation 202 where a number of field-effect transistors (FETs) areformed on a semiconductor substrate. The semiconductor substrate may bea silicon substrate. Alternatively, the substrate may include anotherelementary semiconductor, such as germanium; a compound semiconductorincluding silicon carbide, gallium arsenic, gallium phosphide, indiumphosphide, indium arsenide, and/or indium antimonide; an alloysemiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP,and/or GaInAsP; or combinations thereof. In an embodiment, the substrateis a semiconductor on insulator (SOI) substrate. The SOI substrate mayinclude a buried oxide (BOX) layer formed by a process such asseparation by implanted oxygen (SIMOX), and/or other suitable processes.The substrate may include doped regions, such as p-wells and n-wells. Inthe present disclosure, a wafer is a semiconductor substrate and variousfeatures formed in and over the semiconductor substrate. The wafer maybe in various stages of fabrication and is processed using the CMOSprocess.

In operation 204, a gate contact is formed on a gate in each of theFETs. The gate contacts are formed in typical CMOS processing. Contactsare also formed over and physically connecting to the source/drainregions. In operation 206 a multi-layer interconnect (MLI) is formedover the FETs. The MLI structure may include conductive lines,conductive vias, and/or interposing dielectric layers (e.g., interlayerdielectric (ILD)). The MLI structure provides electrical connection tothe transistor. The conductive lines in various levels may comprisecopper, aluminum, tungsten, tantalum, titanium, nickel, cobalt, metalsilicide, metal nitride, poly silicon, combinations thereof, and/orother materials possibly including one or more layers or linings. Thelinings include adhesion layer, barrier layer, etch stop layer, andanti-reflective coatings. The interposing or inter-layer dielectriclayers (e.g., ILD layer(s)) may comprise silicon dioxide, fluorinatedsilicon glass (FSG), SILK (a product of Dow Chemical of Michigan), BLACKDIAMOND™ (a product of Applied Materials of Santa Clara, Calif.), and/orother insulating materials. The MLI may be formed by suitable processestypical in CMOS fabrication such as CVD, PVD, ALD, plating, spin-oncoating, and/or other processes.

The number of metal layers in the MLI depends on routing needs for theFETs. For simple BioFET devices where little or no analysis andprocessing are performed on the device, fewer metal layers are used, forexample, 3 metal layers. In some embodiments, the BioFET devices processor analyze the measurements, more metal layers are used, for example,four, five, or eight metal layers. The use of more metal layers allowsmore transistors to be used on the device that can perform complex logicoperations with or without additional external input. Further, theresults from the BioFETs can be used as input that triggers furtherdevice operations. In one example, the further device operation may flowthe contents of a microwell toward a more sensitive BioFET or a BioFETwhere a chemical reaction would break up some of the biological content.With additional processing power, a lab-on-a-chip type of device isformed where the output from the device includes results of the analysisinstead of only raw data. For example, the device may determine whethera blood sample contains cancer cells, quantify the cancer cells, andoutput a cancer type. In another example, the device may determine agenetic sequence.

In operation 208, top metal stacks are formed. The top metal stacks havea protective layer on sidewalls of a top metal. As used herein, topmetal stacks are disposed over the MLI. The top metal stacks include anumber of top metals, which are metal plates. The metal plates mayinclude aluminum, copper, or tungsten. Some metal plates are eachindividually connected to the gate contact of a BioFET. Other metalplates are used for signal transmission, such as bonding pads forbonding wires or bumps. FIG. 2B is a process flow diagram of the varioussteps of operation 208 to form the top metal stacks.

In operation 251 of FIG. 2B, an underlayer is deposited on a planarizedsurface over a semiconductor substrate. The planarized surface includesexposed metal vias from the MLI embedded in an intermetal dielectric(IMD). The IMD may be a silicon oxide, silicon nitride, or othercommonly used IMD material including low-k dielectrics containingsilicon oxide with or without pores. The underlayer may includetitanium, titanium nitride, tungsten nitride, tantalum, tantalumnitride, molybdenum, titanium tungsten, or other commonly used copperdiffusion barrier material. The thin underlayer acts as a barrieragainst copper diffusion from the underlying via to the top metal.Excessive diffusion between the copper metal layers create voids andnegatively affect the conductivity of the metal layers.

In operation 253 of FIG. 2B, a top metal is deposited on the underlayer.The top metal includes copper and optionally aluminum, as a mixture oralloy. As discussed, the top metal material is prone to corrosion ifexposed to analytes during the BioFET operation. The top metal is usedunder the microwells as well as for signal routing and externalcontacts.

In operation 255, an antireflective layer is deposited on the top metal.The antireflective layer (ARL) may be a titanium nitride, a siliconoxynitride, or other commonly used antireflective material. Because thetop metal is highly reflective, without an ARL the top metal cannot beaccurately patterned and etched using a lithography process.

FIG. 3 is a cross sectional diagram of a workpiece 300 including the topmetal layers 301 deposited in operations 251, 253, and 255 over a metallayer 303 in the MLI. The top metal layers 301 are deposited on theplanarized surface 305 and includes the underlayer 307, the top metal309, and ARL 311.

Referring to FIG. 2B, in operation 257 a first photoresist is depositedand patterned over the ARL. The first photoresist is patterned to be anetch mask to etch the top metal layers. A typical lithographic processis used to deposit the photoresist, cure the photoresist, expose thephotoresist to patterned light, and develop the photoresist to create afirst pattern. In operation 259, the first pattern is etched through theunderlayer to form separate stacks of the underlayer, top metal, andARL. An etch process removes the ARL, the top metal, the underlayer, anda portion of the underlying IMD to separate the various top metal layersinto top metal stacks. A portion of the underlying IMD is also removedto ensure complete removal of conductive material between the top metalstacks. The etch process may be a dry etch. The dry etch may use achlorine based or a fluorine based etchant in a plasma process. In oneembodiment, the etch process utilizes an end point system where the etchprocess detects an end point material, for example, IMD material, andsignals that the end point for the etch has been reached. At the endpoint, the etch process continues for a defined duration to over etch anadditional amount of material to ensure complete removal of conductivematerial between the top metal stacks.

FIG. 4 is a cross sectional diagram of a workpiece 400 including fourmetal stacks 401 over the metal layer 303. Each metal stack 401 includesan ARL 311, a top metal 309, and an underlayer 307. As shown, a portionof the IMD 403 between the metal stacks 401 is also removed to ensurecomplete removal of the underlayer 307. While the top metal 309sidewalls in FIG. 4 show a slope, according to some embodiments the etchmay be performed such that the sidewalls are nearly vertical.

In operation 261 of FIG. 2B, a protective layer is deposited to covertop and the sidewalls of each metal stack 401. The protective layer isdeposited using CVD or PVD processes and is conformal to the metalstacks formed in operation 259. The protective layer covers thesidewalls of the top metal deposited in operation 253 and patterned inoperation 257. The protective layer may be a copper diffusion barriermaterial such as titanium, titanium nitride, tungsten nitride, tantalum,tantalum nitride, molybdenum, titanium tungsten, or other commonly usedcopper diffusion barrier material. Depending on the method use fordeposition, a top of the metal stack may be deposited thicker thansidewalls of the metal stack. In one example, titanium nitride isdeposited and has a thickness of 500 angstroms or more, for example,about 500 angstroms to about 1000 angstroms over the metal stack and athickness of 250 angstroms or more, for example, about 300 angstroms onthe sidewalls of the metal stack. The protective layer is also depositedin a bottom of the trench areas between the metal stacks. According tovarious embodiments, the thicknesses deposited in each area may varydepend on the aspect ratio of the trenches and the method of deposition.For example, using a PVD process, the top of metal stack and the bottomof low aspect ratios may have a thicker film than at the sidewalls. FIG.5 is a cross sectional diagram of a workpiece 500 including four metalstacks 401 with a protective layer over all of the metal stacks and inthe bottom of trenches 503.

Referring back to FIG. 2B, in operation 263 the protective layer isseparated according to the pattern used to form the metal stacks inoperation 259. In other words, the protective layer at the bottoms oftrenches 503 is removed. In some embodiments, the protective layer isremoved by dry etching without using a patterned etch mask. The dry etchprocess is tuned to bias an etching plasma toward horizontal surfaces ofthe workpiece. By directing an etching plasma downward in the trench,more material is removed from the bottom of the trench than thesidewalls. While some protective layer material would also be removedfrom the top of the metal stack, as long as the top metal is not exposedby the etching process, the top metal would remain protected even if theprotective layer thickness over the top metal is reduced. The amount ofprotective layer removed from the top of the metal stack can beminimized by using an end point detection technique to detect when theIMD material underlying the protective layer is removed. As discussed,once the IMD material is detected, the etch may progress for a specifiedduration to ensure removal of all of the protective material from thetrench bottoms. Such end point detection can also monitor whether anytop material is exposed and etched by detecting the presence of copper.

In some embodiments, the protective layer is removed by dry etchingusing the same photomask used in operation 257 to pattern the firstphotoresist. The exposure operation during the patterning may beadjusted to increase or decrease the size of the pattern developed. Forexample, when using a positive photoresist, a smaller intensity of lightmay be directed through the photomask to reduce the amount of thephotoresist developed, thereby decreasing the opening such that aportion of the sidewall is protected. FIG. 6 is a cross sectionaldiagram of a workpiece 600 including a patterned photoresist 601covering the sidewalls and top of the top metal stacks 603. The patternof the photoresist 601 may be created using the same photomask using inoperation 257 by changing the light intensity or other parameters. Incertain embodiments, a different photomask may be used. The photoresist601 is an etch mask protecting the top portions and sidewalls of the topmetal stack 603. The protective layer 501 on the bottom of trenches 503are removed by dry etch or wet etch. After the protective layer 501 isseparated according to the pattern, the photoresist is removed, as shownin FIG. 7.

FIG. 7 is a cross sectional diagram of a work piece 700 including topmetal stacks 603 over a metal layer 303 and IMD 403. Each of the topmetal stacks include an underlayer 307, a top metal 309 over theunderlayer 309, an ARL 311 over the top metal 309, and a protectivelayer 501 over the ARL 311 and peripherally surrounding the top metal309 covering the sidewalls. After operation 263, the top metal stack isformed and the process refers back to FIG. 2A.

In operation 210 of FIG. 2A, a passivation layer is formed over the topmetal stacks. The passivation layer is deposited over the top metalstack and between adjacent top metal stacks. The passivation layer is adielectric material deposited using CVD processes and may be siliconoxide, silicon nitride, or other commonly used passivation material.FIG. 8 is a cross-sectional view of a portion of a workpiece 800 afteroperation 210 of FIG. 2A. The workpiece 800 includes a passivation layer801 over a top metal stacks 603. The passivation layer fills the trenchareas between the top metal stacks 603, including any openings in theIMD 403 removed when layers above are over etched.

In operation 212 of FIG. 2A, microwells are etched in the passivationlayer to expose some top metal stacks. A layer of photoresist ispatterned to a width. The width may be about the same as a top width ofthe top metal stack or larger than the top width of the top metal stack.Using the patterned photoresist as an etch mask, openings are etched inthe passivation layer to expose some metal plates. The etch process maybe a wet etch or a dry etch. In a wet etch, an ethant is selected thatstops at the protective layer. For example, M2 acid (HNO₃+CH₃COOH+H₃PO₄)would stop a titanium nitride protective layer while etching thepassivation layer material over the top metal stack. In a dry etch, aportion of the protective layer may be removed along with thepassivation layer. FIG. 9 is a cross-sectional view of a portion of aworkpiece 900 after operation 212. Openings 901 and 903 are etched inthe passivation layer 801 to expose portions of the top metal stack 603,respectively. The workpiece 900 is separated into different regions 905,907, and 909 to show different processing scenarios. Region 905 includesan opening 901 that is aligned with the top metal stack 603. A portionof the protective layer 501 on the top metal stack is shown to beremoved during the etch process. Some passivation material 801 adjacentto the top metal stack 603 may also be removed during the etch processwhen the width being etched is larger than the top width of the topmetal stack 603. Region 907 includes an opening 903 that is notperfectly aligned with the top metal stack 603. As result, the openingexposes some sidewall of the top metal stack 603 only on one side.Additional passivation material 801 is also removed from one side of thetop metal stack 603. Depending on the etch selectivity and materialsused for the passivation layer 801 and protective layer 501, in someembodiments, the protective layer 501 is not removed during the etchprocess, as shown in region 907. However, the dimensions of the openingis such that such misalignment does not cause the opening to exposeadjacent top metal stacks. The top metal stack 603 in region 909 is notexposed by an opening. The top metal stack 603 in region 909 is not usedas a floating gate of a BioFET.

Referring back to FIG. 2A, in operation 214 a sensing layer is depositedin the microwells and over a field between the microwells. The sensinglayer is deposited using spin-on coating, CVD or PVD processes havinggood coverage for the sidewalls of the microwells. In some embodiments,an atomic layer deposition (ALD) process is used to conformally coat thebottom and sidewalls of the microwells and the field between themicrowells. The deposition process is selected so that even entrenchedsidewall profiles can be conformally coated. The sensing layer may betitanium nitride, tungsten, a high-k dielectric such as aluminum oxide,lanthanum oxide, hafnium oxide, and tantalum oxide. The sensing layer isdeposited to a sufficient thickness so that electrical signalsrepresenting conditions of the analyte in the microwells can betransmitted to the gate below. The sensing layer may bind directly tobiomolecules in the test sample or indirectly through a surfacetreatment or bioreceptors. The sensing layer may further includeself-assembled monolayers (SAM) and a support medium such as hydrogel.FIG. 10 is a cross-sectional view of a portion of workpiece 1000 afteroperation 214. A sensing layer 1001 is deposited over the wafer and inthe microwells 901 and 903. Depending on the deposition process, thesensing layer may not have uniform thickness in the field portionbetween microwells and the sidewalls in the microwells. Completesidewall coverage ensures that more binding sites are available on thesensing layer for the biomolecules.

Referring back to FIG. 2A, in operation 216 at least the field portionof the sensing layer is removed. Removing the field portion of thesensing layer isolates the sensing layer for different BioFETs from eachother and prevents signal cross talk. Operation 216 includes a number ofsteps. A photoresist layer is deposited on the wafer and planarized. Thephotoresist is then patterned to form openings to expose the sensinglayer on field areas between the microwells. The photoresist may beetched with an end point detection to ensure only the variousphotoresist layers are removed. Exposed sensing layer is removed fromfield areas depending on the material. For certain metal sensing layers,for example, titanium nitride, high-k dielectric material (aluminumoxide, lanthanum oxide, hafnium oxide, tantalum oxide, etc), a selectiveetch is performed to remove the sensing layer on the field areas withoutdamaging microwell sidewalls. Then the remaining photoresist over themicrowells is removed by ashing. In some embodiments, the photoresistfor operation 216 has an opposite exposure from the photoresist foroperation 212. Using the opposite exposure photoresist allows the samephotomask to be used for both operations. For other sensing layers, forexample, hydrogel, a selective chemical mechanical polishing (CMP)process is used without using a photoresist. The selective CMP removesthe hydrogel in the field areas between the microwells without damagingthe hydrogel in the microwells.

FIG. 11 is a cross-sectional view of a portion of workpiece 1100 afteroperation 216. Sensing layers 1001A and 1001B cover the sidewalls andbottoms of microwells 901 and 903, which are isolated from each other.When the photoresist is etched back, the corners of the microwells 901and 903 are exposed, allowing a wet etchant to remove the sensing layerfrom upper portions of the microwell sidewalls. When a CMP process or awet etch is used to remove the sensing layer from the field areas, somesensing layer from upper portions of the microwell sidewalls may also beremoved. As shown, a small edge portion of the sensing layers 1001A and1001B are removed from the lips of the microwells 901 and 903 by theselective metal etching process. In other embodiments, the sensinglayers 1001A and 1001B completely covers the sidewalls of the microwells901 and 903. The photoresist may completely cover the edges of thesidewalls during a dry etch to remove the sensing layer from the fieldareas. When a dry etch is used to remove the sensing layer from thefield areas, the sidewalls of the microwells may remain completelycovered by the sensing layer.

Referring back to FIG. 2, in optional operation 218, openings are formedto expose a second subset of the top metal stacks. The second subset ofthe top metal stacks is the bond pads used for externally connecting theBioFET device. The opening is formed by first depositing and patterninga photoresist layer over the wafer and etching through the pattern. Inthe same etch operation or a different etch operation, a protectivelayer and an antireflective layer is removed in the exposed portions ofthe second subset of the top metal stacks in operation 220. The etchoperation may also remove a portion of the top metal in the top metalstack.

FIG. 12 is a cross-sectional view of a portion of workpiece 1200 afteroperation 218 and 220. Opening 1201 is formed in region 909 to exposetop metal 309 as a bond pad. In some embodiments, the operations 216,218 and 220 may be partially combined to reduce the number of processes.For example, photoresist material in the microwells 901 and 903 may beashed with the photoresist material from operation 218 and 220.

Referring back to FIG. 2A, in operational operation 222, contacts may beformed between the top metal and the second subset of the top metalstacks and a package terminal. In some embodiments, wires are bonded tothe bond pads to transmit power and signal. In other embodiments,conductive members such as bumps and pillars are used to externallyconnect the BioFET device.

In addition to the processes described in association with FIG. 2A, thesensing layer surface may be treated. The treatment may includedepositing a chemical to render the surface hydrophilic or hydrophobic.In some cases, the treatment may modify the surface to have certainconductance or magnetic properties. The treatment may also includedepositing a support medium for attaching receptors. For example,hydrogel or agar may be deposited to the sensing surface.

In some embodiments, BioFET device includes a microfluidic structureover the microwells. The microfluidic structures may include micropumpsand valves and magnetic material or ferromagnetic material formagnetophoresis, metals for electrophoresis, electro wetting ondielectric (EWOD) or particular dielectric material fordielectrophoresis. The microfluidic structure may also electricallyconnect to the various bond pads adjacent to the microwells. Themicrofluidic structure has a bottom that seals the field area betweenadjacent microwells and provides channels for flowing reagents and testsamples. The microfluidic structure may be transparent or partiallytransparent to allow observation of the reactions. In other embodiments,the microwells on the BioFET device is accessed from the top without acover. Microfluidic channels may be formed directly in the passivationlayer.

FIG. 13 is a flow chart of an embodiment of a method 1300 of using aBioFET device according to one or more aspects of the presentdisclosure. In operation 1302, a BioFET device is received. The BioFETdevice is as described in various embodiments of the present disclosure.The BioFET device has a plurality of microwells, wherein a bottomsurface area of the microwell is the same or larger than a top surfaceof the top metal stack under the microwell. The BioFET device alsoincludes a multi-layer interconnect (MLI) connecting the top metal stackto one or more transistor gates. In optional operation 1304, a surfaceof the microwells is treated. In some embodiments, the BioFET device isreceived with the treatment already performed. In other embodiments, thetreatment is performed before after receipt of the BioFET device. Thetreatment operation may include labeling, or functionalizing, thesensing layer with certain chemicals or biomolecules as receptors. Thereceptors may be enzyme, antibody, ligand, peptide, nucleotide, cell ofan organ, organism or piece of tissue is provided or bound on thesensing layer for detection of a target biomolecule. For instance, todetect ssDNA (single-stranded deoxyribonucleic acid), the sensing filmmay be functionalized with immobilized complementary ssDNA strands.Also, to detect various proteins such as tumor markers, the sensing filmmay be functionalized with monoclonal antibodies. The receptors may be apart of self-assembled monolayer (SAM) of molecules. The SAM may havehead groups of silane groups, silyl groups, silanol groups, phosphonategroups, amine groups, thiol groups, alkyl groups, alkene groups, alkynegroups, azido groups, or expoxy groups. The receptors are attached tothe head groups of SAM.

In operation 1306 of method 1300, a test sample is loaded in the BioFETdevice. The test sample may be in a carrier medium. In some embodiments,the test sample is bound to a carrier bead. In other embodiments, thetest sample is suspended in a fluidic medium. The loading operationflows the test sample to various microwells where they are bounddirectly or indirectly to the sensing layer.

In operation 1308, a reagent is flowed in the BioFET device to themicrowells. The reagent reacts with some or all of the test samples inthe microwells. The existence of reaction or the extent of the reactionis recorded by measuring the current through the source and drain of theBioFET in operation 1310. Several measurements of the current may bemade at different times. For example, a blank measurement may be made toestablish the baseline with deionized water. Another measurement may bemade after the test sample is loaded to establish a second baseline. Oneor more measurements may be made to record the change in current duringthe reagent flow and residence in the BioFET device. In someembodiments, FET devices operate in linear region for detection. In someembodiments, FET device operate in saturation region for detection.

In operation 1312, the measurement is analyzed. The measurement may beoutputted by the BioFET device to a computer or a processor to analyzethe signals. In some embodiments, an analog signal is first converted toa digital signal. The data may be analyzed by a processor running asoftware program or by a user. In some embodiments, the measurement isanalyzed on board the BioFET device.

The BioFET device may be a single use or a multiple use device. Inoptional operation 1314, the BioFET device is flushed to remove thereagent from the microwells and operations 1308 to 1312 repeated with asecond reagent. The test sample remains in the BioFET device asdifferent reagents are cycled through. This process may be used toidentify an unknown substance. By recording reactions using differentreagents, the identity of an unknown substance may be narrowed down.This process may be used to perform DNA sequencing. For example, a testsample of strands of DNA may be loaded into BioFET device. The strandsmay be amplified in each microwell to form a colony. By sequentiallyadding reagents containing different nucleobases and measuring reactionsin each microwell, the identity of the strand in each microwell may befound.

In one aspect, the present disclosure pertains to a biologicalfield-effect transistor (BioFET) device that includes a substrate and anumber of BioFETs. The BioFET includes a microwell having a bottom andsidewalls, a number of top metal stacks under the microwells, each ofthe top metal stacks being under one microwell, and one or moretransistors, wherein a gate of each of the one or more transistors isconnected to one of the plurality of top metal stacks throughintervening metal layers. The bottom is a sensing layer and at least aportion of the sidewalls is a sensing layer. Each top metal stacksinclude an underlayer, a top metal over the underlayer, and a protectivelayer over and surrounding the top metal; and the underlayer and theprotective layers are conductive.

In another aspect, the present disclosure pertains to a method offorming a BioFET device. The method includes forming a number of FETs ona semiconductor substrate, forming a gate contact on the gate structurein each of the FETs, forming one or more metal interconnect layers overthe FETs, forming a number of top metal stacks having a protective layeron sidewalls of a top metal, forming a passivation layer over the topmetal stacks, etching microwells in the passivation layer to expose asubset of the top metal stacks, depositing a sensing layer in themicrowells and over a field portion between the microwells, and removingat least the field portion of the sensing layer. The FETs each includesa gate structure formed on a first surface of the semiconductorsubstrate and a channel region. The bottom surface area of themicrowells is larger than a top surface of the subset of the pluralityof top metal stacks. The sensing layer fills any openings between thesubset of the plurality of top metal stacks and the passivation layer.

An advantageous feature of illustrated embodiments may include abiological field-effect transistor (BioFET) device, comprising asubstrate, and a plurality of BioFETs. Each BioFET comprises a pluralityof microwells having a bottom and sidewalls, wherein the bottom is asensing layer and at least a portion of the sidewalls is a sensinglayer, and further comprises a plurality of top metal stacks under theplurality of microwells, each of the plurality of top metal stacks beingunder one microwell. Each of the plurality of the top metal stacksincludes an underlayer, a top metal over the underlayer, and aprotective layer over and surrounding the top metal and wherein theunderlayer and the protective layers are conductive. Each BioFET furtherincludes one or more transistors, wherein a gate of each of the one ormore transistors is connected to one of the plurality of top metalstacks through intervening metal layers.

Another advantageous feature of illustrated embodiments may include adevice, comprising a substrate and a BioFET structure. The BioFETstructure includes a microwell having a bottom and sidewalls, whereinthe bottom and at least a portion of the sidewalls form a sensing layer.A top metal stack is under the microwell, the top metal stack includinga major conductive feature sandwiched between a conductive underlayerand a conductive protective layer, the conductive protective layer beingconformal to a top surface of the major conductive feature. The devicefurther includes a transistor, electrically connected to the majorconductive feature through one or more intervening metal layers.

Yet another advantageous feature of illustrated embodiments may includea device comprising a transistor, and an interconnect structureelectrically connected to the transistor. The interconnect structureincludes a top metal stack that has a conductive underlayer, aconductive line atop the underlayer, and a conductive protective layeratop the conductive line and extending along sidewalls of the conductiveline. The device further includes a passivation layer atop the top metalstack having a microwell formed therein, wherein sidewalls of themicrowell are formed from the passivation layer and a bottom of themicrowell is formed from the conductive protective layer. A sensinglayer lines the bottom and at least partially lines the sidewalls of themicrowell.

In yet another aspect, the present disclosure pertains to a method ofsensing bio-reactions. The method includes receiving a BioFET device asdisclosed in the present disclosure, loading a test sample in the BioFETdevice, flowing a reagent in the BioFET device to the plurality ofmicrowells, measuring a change in a transistor current corresponding toeach of the plurality of microwells, and analyzing the measurement.

In describing one or more of these embodiments, the present disclosuremay offer several advantages over prior art devices. In the discussionof the advantages or benefits that follows it should be noted that thesebenefits and/or results may be present is some embodiments, but are notrequired in every embodiment. Further, it is understood that differentembodiments disclosed herein offer different features and advantages,and that various changes, substitutions and alterations may be madewithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A biological field-effect transistor (BioFET)device, comprising: a substrate; and a plurality of BioFETs, each BioFETcomprising: a plurality of microwells having a bottom and sidewalls,wherein the bottom and at least a portion of the sidewalls is a sensinglayer; a plurality of top metal stacks under the plurality ofmicrowells, each of the plurality of top metal stacks being under onemicrowell, wherein each of the plurality of the top metal stacksincludes an underlayer, a top metal over a top surface of the underlayerand not extending below the top surface of the underlayer, and aprotective layer over and along a sidewall of the top metal, wherein aportion of the protective layer extends lower than an uppermost surfaceof the underlayer and wherein the underlayer and the protective layersare conductive; and one or more transistors, wherein a gate of each ofthe one or more transistors is connected to one of the plurality of topmetal stacks through intervening metal layers.
 2. The BioFET device ofclaim 1, further comprising a metal pad having a top surface below aplane of top surfaces of the plurality of top metal stacks.
 3. TheBioFET device of claim 1, wherein the each of the plurality of top metalstacks also includes an antireflective layer over the top metal andunder the protective layer.
 4. The BioFET device of claim 1, wherein athickness of the protective layer on sidewalls of the top metal stacksis greater than 200 angstroms.
 5. The BioFET device of claim 1, whereina thickness of the protective layer on sidewalls of the top metal stacksis about 300 angstroms.
 6. The BioFET device of claim 1, wherein theprotective layer comprises titanium nitride.
 7. The BioFET device ofclaim 1, further comprising: a fluidic channel fluidly connecting themicrowells between the plurality of BioFETs.
 8. The BioFET device ofclaim 1, wherein a bottom surface area of each of the plurality ofmicrowells is larger than a top surface of the top metal.
 9. The BioFETdevice of claim 1, wherein the sensing layer comprises a high-kdielectric.
 10. The BioFET device of claim 1, wherein the sensing layeris disposed between a top portion of the plurality of top metal stacksand a passivation layer, wherein the passivation layer is disposedbetween each of the plurality of BioFETs.
 11. The BioFET device of claim10, wherein the sensing layer does not cover a top surface of thepassivation layer.
 12. A device, comprising: a substrate; and a BioFETstructure including: a microwell having a bottom and sidewalls, whereinthe bottom and at least a portion of the sidewalls form a sensing layer;a top metal stack under the microwell, wherein the top metal stackincludes a major conductive feature sandwiched between a conductiveunderlayer and a conductive protective layer, the conductive protectivelayer being conformal to a top surface and one or more sidewalls of themajor conductive feature and extending to an uppermost surface of theunderlayer, the major conductive feature not extending below theuppermost surface of the underlayer; and a transistor, electricallyconnected to the major conductive feature through one or moreintervening metal layers.
 13. The device of claim 12, wherein themicrowell is formed in a passivation layer formed over the top metalstack.
 14. The device of claim 12, wherein the protective layercomprises titanium nitride.
 15. The device of claim 12, furtherincluding an anti-reflective coating sandwiched between the majorconductive feature and the conductive protective layer.
 16. The deviceof claim 12, wherein the bottom of the microwell is substantiallyaligned with the major conductive feature.
 17. A device comprising: atransistor; a interconnect structure electrically connected to thetransistor, the interconnect structure including a top metal stackcomprising: a conductive underlayer, a conductive line atop theunderlayer and not along sides of the underlayer, and a conductiveprotective layer atop the conductive line and extending along sidewallsof the conductive line; a passivation layer atop the top metal stackhaving a microwell formed therein, wherein sidewalls of the microwellare formed from the passivation layer and a bottom of the microwell isformed from the conductive protective layer; and a sensing layer liningthe bottom and at least partially lining the sidewalls of the microwell,wherein an uppermost surface of the passivation layer extends above thesensing layer.
 18. The device of claim 17, wherein the conductiveprotective layer comprises titanium nitride.
 19. The device of claim 17,wherein the interconnect structure includes a plurality of conductiveelements vertically stacked atop one another and electricallyinterconnected by conductive vias.
 20. The device of claim 17, whereinthe top metal stack is vertically aligned to the transistor.
 21. Thedevice of claim 17, wherein the sensing layer extends from a top to abottom of the sidewalls.
 22. The device of claim 17, wherein a portionof the protective layer extends lower than an uppermost surface of theunderlayer.